Filson's description of the fabric:
Woven to our unique specifications, our heavyweight 24-oz. Mackinaw Wool provides greater resilience, more natural insulating qualities, increased breathability and superior water repellency. It comes from a small herd of hardy sheep in the northwestern United States and Canada. Our wool will absorb as much as 30% of its own weight in moisture without becoming damp or clammy, allowing it to retain body warmth even when very wet. The fibers can even be bent and flexed 20,000 times without breaking. We’re certain that there is no finer wool than Filson’s 100% virgin Mackinaw Wool.